Cam phasers are well known in the automotive art as elements of systems for reducing combustion formation of nitrogen oxides (NOX), reducing emission of unburned hydrocarbons, improving fuel economy, and improving engine torque at various speeds. As is known, under some operating conditions it is desirable to delay or advance the closing and opening of either the intake valves or the exhaust valves or both, relative to the valving in a similar engine having a fixed relationship between the crankshaft and the camshaft.
Cam phasers employ a first element driven in fixed relationship to the crankshaft and a second element adjacent to the first element and mounted to the end of the camshaft in either the engine head or block. The first element is typically a cylindrical stator mounted coaxially to a crankshaft-driven gear or pulley and having a plurality of radially-disposed chambers separated by inwardly-extending radial lobes and the second element is a vaned rotor mounted to the end of the camshaft through an axial bore in the stator and having a vane disposed in each of the stator chambers such that limited relative rotational motion is possible between the stator and the rotor. The chambers are sealed typically by front and rear face seals of the stator. The apparatus is provided with suitable porting so that hydraulic fluid, for example, engine oil under engine oil pump pressure, can be brought to bear controllably on opposite sides of the vanes in the chambers. Control circuitry and valving, commonly a multiport spool valve, permits the programmable control of the volume of oil on opposite sides of each vane to cause a change in rotational phase between the stator and the rotor, in either the rotationally forward or backwards direction, to either advance or retard the opening of the valves with respect to the position of the pistons in the cylinders.
Cam phaser designs heretofore have faced two powerful and antithetical needs: compact size and high torque capacity.
Regarding small size, many engine applications require the addition of a cam phaser unit within the envelope of an existing engine design which may have been in production for several years and which gives no special consideration to space for such a unit in the vicinity of the camshaft end. Thus, to be useful with as many engine designs as possible, a cam phaser should occupy as little volume as possible.
Regarding high torque capacity, a cam phaser must be capable of generating sufficient rotational torque between the stator and rotor to drive the advancing and retarding of valve opening and closing, which average torque for representative engines can be about 3 Nm. The rotational torque that a cam phaser is capable of generating depends on several variables including the operating temperature of the engine, the operating age of the engine, the viscosity of the oil, and numerous other known engine factors. Therefore, for smooth engine operation and rapid response under all anticipated oil pressure and use conditions, the torque capacity of a cam phaser should be substantially greater than the average torque of 3 Nm for representative engines.
Torque capacity is the product of force applied at a distance from an axis of rotation. The torque capacity (T.sub.1) of a specific cam phaser can be expressed in terms of available oil pressure (P) and volume displacement per radian (V) as follows: EQU T.sub.1 =PV (Eq. 1)
A volume parameter, referred to herein as the Phaser Envelope (PHE), is defined herein as pi times the square of the stator diameter D times the axial length L of the phaser, divided by 4, and represents the cylindrical volume required to contain the phaser: EQU PHE=(.pi.D.sup.2 L)/4 (Eq. 2)
The radial dynamic load imposed by the engine on the timing sprocket or pulley of the cam phaser is quite large and requires a substantial axial length of bearing between the sprocket and the camshaft to sustain the load. In prior art cam phasers, such a bearing is provided adjacent to, and axially displaced from, the stator/rotor hydraulic components of the phaser, thereby extending substantially the overall axial length of the phaser, the space required in the engine envelope to accommodate the phaser and, most significantly, the cylindrical volume or phaser envelop (PHE) required to contain the phaser. The PHE, in cm.sup.3, for prior art cam phasers having axially displaced bearings is typically in the range of 250 to 300. In a newer compact cam phaser design, disclosed in U.S. Patent to Lichti, et al titled "Diametrically Compact Cam Phaser", assigned to the assignee hereof, bearing Ser. No. 09/388,103, the cam phaser has a PHE of 190 cm.sup.3 while still maintaining a torque capacity (T.sub.1) of 4.8 Nm at 20 psi oil pressure. However, in the Diametrically Compact Cam Phaser, because the bearing is displaced axially away from the stator/rotor hydraulic components, approximately 36% of the unit's total PHE is used up in providing support for the radial load of the chain drive.
What is needed is a compact cam phaser having an axial length significantly less than that of prior art cam phasers such that the Phaser Envelope can be substantially smaller at no significant sacrifice in bearing load capability or torque capacity.